From U.S. Pat. No. 4,756,240 A there is known a piston bolt of this kind with an axial opening, wherein at least one reinforcement rib is arranged on an inner surface of the opening. This is supposed to achieve an optimization of the loads.
The piston bolt constitutes the force transmitting connection element between piston and connecting rod and is stressed by the gas and mass forces acting on it with bending as well as transverse ovalization, and also shearing in certain cross sections, under which both the piston bolt and the bolt bosses in the piston and in the connecting rod are elastically deformed. The contact surfaces of piston bolt and bolt bosses, often being cylindrical and parallel to each other in the load-free state, become curved and canted, so that local surface pressure peaks can occur. Together with the tensile stress acting in the circumferential direction of the bolt bosses, this constant stressing can exceed the fatigue strength of the piston material and result in fatigue cracks in the apex of the bolt bosses, so-called, boss gap cracks. To prevent this, low-deformation piston bolts are required, that is, bolts with large wall thickness, but this in turn runs counter to the desire for the lowest possible masses to be moved.
In general, it should be noted that, while the bending resistance is improved with axially running reinforcement ribs, this runs counter to the transverse ovalization of the bolt. On the contrary, with reinforcement ribs arranged in a ring, the resistance to transverse ovalization is improved, but the bending resistance is reduced. Thus, in both instances, only one of the two properties is improved, while the other one is worsened.